Conventional stroke sensors of cylinder apparatuses made of a composite material comprise a stroke sensor disclosed in JP, A, 63-238415 which employs the magnetostriction effect. This stroke sensor has a cylinder tube comprising an inner cylinder made of a nonmagnetic material and an outer cylinder formed by winding a fiber material consisting of a strand of continuous filaments impregnated with a resin on the outer periphery of the inner cylinder. A position indicating magnet is provided on the piston which is slidably disposed in the cylinder tube, and an elongated sensor body, which employs the magnetostriction effect, is interposed between the inner and outer cylinders of the cylinder tube. The sensor body comprises a thin elongated tube containing an amorphous ribbon, a drive coil wound at one end of the thin elongated tube for inducing the magnetostriction phenomenon in the amorphous ribbon and generating an ultrasonic wave when a pulsatile input current is applied to the thin elongated tube, and a detection coil wound over a predetermined length of the thin tube for detecting as a detection signal the induced electromotive force generated by the inverse magnetostriction phenomenon when the ultrasonic wave passes through a position of the amorphous ribbon, which corresponds to the position indicating magnet. Because the time taken from the application of the pulsatile input current to the detection of the detection signal depends upon the piston stroke, the piston stroke can be detected by measuring the time.
On the other hand, since the sonic speed of the ultrasonic wave generated depends upon the temperature of the apparatus, an increase in oil temperature of the cylinder apparatus causes changes in the sonic speed of the ultrasonic wave and the above-described time and thus causes error in the measurement. JP, A, 63-238415 therefore proposes temperature compensation methods for correcting the measurement error produced. In a first method, a fixed compensating magnet is embedded at a predetermined position between the inner and outer cylinders, the induced electromotive force, which is produced by the inverse magnetostriction phenomenon when the ultrasonic wave passes through the fixed magnet, is detected as a second detection signal, and the distance between the drive coil and the fixed magnet is used as a reference distance for temperature compensation so that the detection signal generated by the position indicating magnet is corrected by using the reference distance and the time taken from the application of the pulsatile input current to the detection of the second detection signal. In a second method, one of two magnets, which are disposed at both axial ends of the piston, is also used as a magnet for indicating the position of the piston, the induced electromotive forces obtained from the two magnets are detected as detection signals, and the distance between the two magnets is used as a reference distance for temperature compensation so that the detection signal generated from the position indicating magnet is corrected by using the time difference between the two detection signals and the reference distance.
However, the above-mentioned temperature compensation methods have the following problems:
In the first method, although the distance between the detection coil and the fixed compensating magnet is used as the reference distance, the reference distance cannot be measured unless the positions of the detection coil and the fixed magnet are determined. Since the drive coil itself has a certain length, the position of the drive coil cannot be correctly determined. Further, the fixed compensating magnet is disposed on the outer surface of the inner cylinder formed, and the outer cylinder is then formed thereon. However, the position of the fixed magnet is easily shifted when a fiber material is wound for forming the outer cylinder. In addition, since both the drive coil and the fixed magnet are buried in positions between the inner and outer cylinders, the positions of the drive coil and the fixed magnet cannot be seen from the outside after the cylinder tube has been completely formed. For the above reasons, the distance between the detection coil and the fixed magnet cannot be accurately measured, and the accuracy of temperature compensation thus deteriorates.
In the second method, although the distance between the two magnets provided on the piston is used as the reference distance, the two magnets cannot be seen from the outside after the piston has been incorporated in the cylinder tube. Further, the accuracy of compensation increases with an increase in the reference distance. In this method, however, because the distance between the two magnets is short due to the short length of the piston itself, it is impossible to ensure that the reference distance has a sufficient length. For the above reasons, this method also has a problem with respect to the low accuracy of temperature compensation.
Examples of prior art related to a cylinder, which is formed by winding a strand of continuous filaments impregnated with a resin, include JP, A, 63-166522 and 63-249628.
It is an object of the present invention to provide a cylinder apparatus made of a composite material comprising a stroke sensor which enables an improvement in accuracy of temperature compensation for a change in environmental temperature such as the oil temperature or the like and which enables the precise measurement of the piston stroke.